The present invention relates to a method for operating an internal combustion engine having an exhaust gas purification system, with intake air throttling being carried out in order to raise an exhaust gas temperature in a low-load mode and in a traction mode.
DE 44 15 650 C2 discloses a method for operating an internal combustion engine, in which a mass intake airflow is reduced in order to raise an exhaust gas temperature level. The reduction is to take place such that, after a fuel injection, there is a virtually stoichiometric mixture at each operating point of the internal combustion engine. This ensures that the exhaust gas temperature level is always at a maximum value for the instantaneous operating point and a catalytic converter arranged in the exhaust train of the internal combustion engine reaches its activation temperature as quickly as possible. The amount of reduction in the mass intake airflow is to be determined from a characteristic map as a function, inter alia, of the residual oxygen content in the exhaust gas. This is intended to ensure that the mixture can never become too rich even during the nonstationary operation of the internal combustion engine. When the operating temperature of the catalytic converter is already reached, the mass intake airflow is no longer reduced to the same extent as when the internal combustion engine is cold, but, instead, only to an extent such that the catalytic converter is maintained at its optimum operating temperature.
DE 197 49 400 A1 discloses a method for reducing the NOx content in the exhaust gas of a diesel internal combustion engine. In order to avoid a cooling of the exhaust gas in the overrun mode, an efficiency of the diesel internal combustion engine is deliberately impaired. For this purpose, for example, an exhaust gas recirculation rate can be reduced at the expense of a minimization of consumption.
DE 197 31 623 A1 discloses a method for operating a direct-injection diesel engine, in which a rise in the exhaust gas temperature is to be achieved by means of mass intake air throttling.
DE 44 40 833 A1 also discloses a method for operating a diesel engine, in which the exhaust gas temperature can be increased by a reduction in the mass air throughput, for example by means of an exhaust gas turbocharger of variable geometry, intake air throttling, boost pressure cutback or intake air preheating.
Modern internal combustion engines are distinguished by ever increasing efficiency, above all in the part-load range, this applying particularly to direct-injection gasoline and diesel engines. Benefits of a higher efficiency are a lower consumption, lower untreated emissions and therefore increased environmental compatibility. The improved efficiency has an adverse effect on catalytic exhaust gas aftertreatment systems, however, because less exhaust gas energy and therefore a lower exhaust gas temperature are available for heating the exhaust gas aftertreatment systems. The result of this may be that these exhaust gas aftertreatment systems reach the temperature range necessary for optimum pollutant conversion later or even never. Under certain circumstances, because of the poorer convertibility of the exhaust gas aftertreatment systems, this leads to an overall poorer emission behavior in spite of reduced untreated emission values.
In addition, this problem becomes more serious when discontinuous exhaust gas aftertreatment systems, such as NOx storage catalytic converters and/or diesel particle filters, are used. These systems require, for the conversion or removal of the stored exhaust gas constituents, for example NOx, SOx, particles, regeneration phases which, if appropriate are generated actively at regular intervals and are associated, inter alia, with marked rises in exhaust gas temperatures. Among other things, the active introduction of an afterinjection following the actual main combustion may contribute to generating these required exhaust gas temperatures. This afterinjection is converted completely or partially in the combustion space of the engine, and possibly unburned fuel can additionally be burnt on an optionally preceding oxidizing catalytic converter and actively heat the systems to be regenerated. For this purpose, however, such an oxidizing catalytic converter must be kept in the temperature window necessary for converting the introduced fuel. Furthermore, for a reduction in energy and therefore in consumption, it is advantageous to have as high an initial temperature as possible at the commencement of the active heating operation.
In systems consisting of internal combustion engines and exhaust gas aftertreatment systems which generate the necessary regeneration temperature predominantly or even only partially within the engine, the problem arises that there are engine operating states which naturally do not allow any increase in the exhaust gas temperature. This applies particularly to the overrun mode, since this is characterized by a negative engine output torque and therefore, in the overrun mode, no load-generating and therefore also no temperature-generating injection of fuel is carried out.
Since the driving state overrun mode in the motor vehicle may have a substantial fraction of an overall driving profile, it is important to keep the exhaust gas temperatures already reached as high as possible at least as long as possible, and therefore to keep the unavoidable cooling of the exhaust gas in the overrun mode as low as possible. A highly effective possibility of markedly reducing the cooling of the exhaust gas in the overrun mode is a reduction in the fresh air mass passed through the engine. The reduction may be achieved by the activation of various throttle points which may be introduced between the intake tract and the exhaust tract. The opening of an existing exhaust gas recirculation device may also reduce the cooling air throughput through the engine. Such methods operating with intake air throttling are mentioned, for example, in the aforementioned documents.
The known measures for reducing the mass air throughput by throttling actions on the fresh air side not only cause a higher exhaust gas temperature level to be maintained, but also adversely result in a changed response behavior of the vehicle. In particular, when the driver requires load after a throttled overrun phase, the response behavior becomes noticeably poorer.
Alternatively, throttling devices mounted on the exhaust gas side may ensure a reduction in the mass air throughput, but such throttling devices on the exhaust gas side, due to the induced rise in counterpressure, ensure a clearly noticeable braking torque comparable to what is known as the engine brake in motor trucks.
A further alternative for reducing the mass air throughput is to employ a bypass valve system which is mounted on the exhaust gas side and, in overrun phases, diverts exhaust gas having a high fresh air fraction around the actual active exhaust gas aftertreatment systems. However, such systems are complicated in structural terms and are costly.
An object of the present invention is to provide a method for operating an internal combustion engine having an exhaust gas purification system in which a rise in the exhaust gas temperature in low-load and overrun phases is allowed, without the response behavior of the internal combustion engine being noticeably impaired.
For this purpose, according to the invention, a method for operating an internal combustion engine comprising an exhaust gas purification system is provided, intake air throttling being carried out in order to raise an exhaust gas temperature in a low-load mode and a traction mode. An amount of intake air throttling is determined as a function of an expected load demand so that, in the case of an expected unchanged or decreasing load demand, an increased or maximum possible intake air throttling is set, and, in the case of an expected rising load demand, no or reduced intake air throttling is set. The expected load demand is estimated on the basis of operating and ambient parameters of the internal combustion engine.
The invention thus provides a method, by way of which intake air throttling can be carried out a in foresighted way, since, by the use of suitable information systems which, if appropriate, are already installed in the vehicle, the throttling rates can be adapted dynamically to the expected estimated driving behavior. This makes it possible to have temporally markedly higher throttling rates and consequently a markedly reduced cooling of existing exhaust gas temperatures, without any adverse effects on the response or driving behavior.
In particular, by virtue of the invention, the response behavior of the internal combustion engine is markedly improved after an overrun phase with a lowered mass air throughput, corresponding to what is known as throttle operation. Thus, with the present invention, the throttling rates are raised dynamically in operating phases in which no load requirement by the driver is expected, whereas the throttling rates are kept as low as possible in operating phases with a potential load requirement. The estimation of the expected load demand, this corresponding in the vehicle to an estimation of the expected behavior of a driver, is in this case to take place on the basis of sensor information and further information which reproduces current conditions from which certain possible consequential actions can be inferred.
In a development of the invention, the internal combustion engine is provided for a motor vehicle and the expected load demand is estimated as a function of a brake pedal position, of a brake pressure, of a profile of the brake pedal position and/or of a brake pressure profile.
Thus, perfectly reliable evidence of the expected load demand can be obtained, since, in an overrun phase, with the brake pedal depressed, no load requirement by the driver is to be expected. Intake air throttling can thus be increased on the basis of the interlinking of the information “overrun mode” or “low-load mode” and “brake pedal depressed”. For example, the intake air throttling rate may also be increased in proportion to the travel by which the brake pedal is depressed downward. Advantageously, the profile of the brake pedal travel or of the brake pressure is also detected and evaluated, so that, even in the case of the least possible indication of a reduction in braking action, the intake air throttling rate is cut back superproportionally in a speculative way, since, if there is a reduction in the braking action, the expectation that a driver then intends to step on the gas again is justified. In this way, even in the case of a rapid change from brake to accelerator pedal, the rise in the fresh air mass required for ensuring a good response behavior can be initiated in a foresighted way. In order to improve the response behavior further after a reinforced throttling action, a dynamic air mass rise can advantageously be implemented, which, when a potential termination of the braking action by the driver is detected, has already as far as possible reduced the intake air throttling rate.
In a further development of the invention, the internal combustion engine is provided for a motor vehicle and the expected load demand is estimated as a function of a transmission position, in particular linked to a current driving speed.
For example, with a transmission gear not selected, no load requirement of the driver is to be expected. This applies particularly when the vehicle is stationary. In such an idling or lowest-load mode in which only the traction torque of the engine has to be overcome, a maximum possible intake air throttling rate can thereby be set, in order, even in the idling mode, to keep the exhaust gas temperature as high as possible and to achieve a rapid heating of the exhaust gas aftertreatment systems or an only slight cooling of the latter.
In yet another development of the invention, the internal combustion engine is provided for a motor vehicle and the expected load demand is estimated as a function of a measurement of the distance from a vehicle traveling ahead.
For example, information from a radar-assisted distance measuring instrument, what is known as a Distronic, may be used. Thus, for example, the rapid approach to a vehicle traveling ahead causes a marked lowering in the probability of a load requirement by the driver.
In a still further development of the invention, the internal combustion engine is provided for a motor vehicle and the expected load demand is estimated as a function of information on the position and/or location of the vehicle.
For this purpose, for example, information from a navigation system, for example a satellite-assisted global positioning system, may be used. If, for example, the vehicle is just approaching an intersection with traffic lights, the probability of a load requirement of the driver falls, especially when the brake pedal is depressed at the same time. Furthermore, however, for example, information on the current inclination of the vehicle may also be used. Thus, as a rule, no or only a low load requirement is to be expected during downhill travel and in the overrun mode. In this case, terrain information may also be used in a foresighted way, such as, for example, whether the vehicle is just at the start of a downhill stretch.
In a development of the invention, the internal combustion engine is provided for a motor vehicle and the expected load demand is estimated as a function of acceleration spin information.
Information on the acceleration spin, for example on a slippery surface, likewise suggests the absence of a load requirement of the driver, especially when the acceleration spin information is seen in connection with further information from an outside temperature sensor and a rain sensor. In this case, for example, information provided by a driving dynamics control system may also be used. If, for example, the driving dynamics control system brings about a reduction in the drive torque, it may be assumed that there are no load demands on the internal combustion engine.
Suitable operating and ambient parameters for estimating the expected load demand can be delivered by all systems which are capable of being able to forecast a driver's requirement with some degree of certainty. These are, for example, driver assistance systems which can provide information on the nature of terrain, traffic density, driving destinations and the like.